No, it’s not you. I just tested it and it gave me the same error, but then I searched the forum and found that the custom message feature in SHM and Speaker Notify with Sound stopped working last week.
If you want to stick with SHM and/or Speaker Notify with Sound, set the Alarm Track in the Device Settings and use it like an alarm or a switch. Turning the switch or alarm on will play the track specified in the settings.
The other option is to use a community developed SmartApp like either Rule Machine or CoRE. The developer pulled Rule Machine so unless you already had it installed, you will have to use CoRE instead.
Breaking that feature is a pretty big deal so it probably won’t take SmartThings too long to fix it.